Multiple pure tone noise suppression device for an aircraft gas turbine engine

ABSTRACT

Sound suppression means for a gas turbine power plant which includes a fan positioned within a bypass duct surrounding a core engine which drives said fan. An inflatable diaphragm is positioned within an inlet opening of said bypass duct along the inner wall of the casing which surrounds the fan. Means are provided for selectively inflating said diaphragm such that inlet flow along said inner wall can be increased to sonic or near sonic velocity thereby effectively preventing propagation of multiple pure tone noise generated by said fan from said bypass duct.

United States Patent Inventors Appl. No. Filed Patented AssigneeMULTIPLE PURE TONE NOISE SUPPRESSION DEVICE FOR AN AIRCRAFT GAS TURBINEENGINE 8/1960 Nelson 137/l5.1

2,966,028 12/1960 Johnson et a1. 137/l5.1

3,484,847 12/1969 Poole 181/33 (.222)

3,494,380 2/1970 Martin 137/15.1

3,532,100 10/1970 Ward etal. l37/l5.1 FOREIGN PATENTS 1,510,897 12/1967France 181/33 (.222)

Primary Examiner-Robert S. Ward, Jr.

Attorneys-Derek P. Lawrence, Thomas J. Bird, Jr., Lee H.

Sachs, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman 6Claims, 6 Drawing Figs.

U.S. Cl 181/33 HA, ABSTRACT; Sound suppression means for a gas turbine181/33 HD, 137/ 15.1 power plant which includes a fan positioned withina bypass II- duct surrounding a core engine which drives said fan An in.3 flatable diaphragm is positioned within an inlet opening of said Fieldof Search 181/33, b as duct along the inner wall of the casing whichsurrounds -1, 35; 137/151. 15.2; 415/1 19 the fan. Means are providedfor selectively inflating said diaphragm such that inlet flow along saidinner wall can be in- Cmd creased to sonic or near sonic velocitythereby effectively U TED STATES TE preventing propagation of multiplepure tone noise generated 2,763,426 9/1956 Erwin 181/33 (.21) by saidfan from said bypass duct.

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sum 1 0F 2 INVENTORS. ROBERT C. EVANS BY WWLIAM R. MORGAN PATENTEmmv 9IQYI INVENTORS; ROBERT C. EVANS WlLLlAM R. MORGAN facturers before anaircrafl MULTIPLE PURE TONE NOISE SUPPRESSION DEVICE FOR AN AIRCRAFT GASTURBINE ENGINE BACKGROUND OF THE INVENTION gine powered aircraft havecreated noise problems for both the general public and the aviationindustry as well as for engine manufacturers. With the general growth inthe aviation industry has come the generation of noise level standardswhich must be met by aircraft and gas turbine engine manupowered by agas turbineengine will be certified by the Federal Aviation Agency.Thisgeneral growth in the aviation industry. however, has also resultedin a demand for newer and larger and. more powerful gas turbine engines.This demand has resulted in the birth of a new family of high bypass,large diameter turbofan engines capable of producing thrust levels ofgreater than 40,000 pounds. .While experimentation has shown that thenoise levels generated by this new family of turbofan engines are lowerthan those of previous generation engines, a vast amount of effort isstill being expended in an attempt tofurther reduce the noise levelsassociated with suchengines.

In the air intake duct of a gas turbine engine, objectionable soundstravel in the direction opposite that of the airflow and out of theintake duct. Thesesounds originate within the engine and are transmittedthrough the incoming air. If the velocity of the incoming air in theduct is increased to equal or exceed the velocity of sound at allincoming air conditions across a plane perpendicular tothe inlet ductscenter line at any one place along the length of the duct, thiscondition (known as choked flow) operates as a sound barrier throughwhich undesirable sound waves cannot pass. Such a phenomenon has beenshown to b quite effective with regular gas turbine engines.

An apparatus for providing such a choked flow condition has been shownand claimed. in an,application of John T. Kutney, Ser. No. 1,223,entitled "Choked Inlet Noise Suppression Device for a Turbofan Engine,"and assigned to thesame assignee as the present application. As shown inthe Kutney application, an inflatable diaphragm is positionedaround theinterior wall of a duct surrounding a turbofan engine. This .inflatablediaphragm provides completely choked flow to a passageway between theduct and a contoured surface of a splitter platform which extends from astationary-inlet guide vanes and divides the fan duct into .inner andouter passageways. Such a device has beenshown to efiectively restrictthe transmission of all sound generated within the outer passageway ofthe fan duct.

A Recent developments in the gas turbine manufacturing art have shownthat for some applications the stationary inlet guide vanes shown in theKutney application are unnecessary for successful engine operation. Insuch applications, the inlet guide vanes merely add unnecessary weightand increase the cost of the engine. A Kutney-type noise suppressorwhich chokes the entire inlet flow would have to work on the total inletflow area of an IGV-less engine andwould require large physical changesin the flow path contour. Because of the associated performance losseswith the flow path changes, a Kutney-type suppressor may be impracticalfor such an engine. In contrast to the Kutney device, the subject deviceis designed primarily to reduce multiple pure tone (MPT) noise which isa characteristic of all high bypass ratio turbofan engines operating atand above fan rotor sonic tip relative Mach numbers, particularlyIGV-less front fan engines.

There are two current theories on the generation and propagation of MP1noise, which is primarily pure tone noise measured in the engine farfield, audible. and at high enough levels to be annoying to the humanear if sufficient sound suppression is not applied. The first theory maybe generally designated the shock wave theory. As the fanwhe el speedincreases, a point is reached (in the region of tip relative Mach numberequal to one) where shock waves start to form at the fan blade tips.These shock waves are transmitted upstream along the-inlet duct wall atfrequencies which are per revolution multiples of the wheel speed, i.e.,NXr.p.m./60'where N is an integer l, 2, 3, etc. These frequencies arenormally less than the blade passing fundamental frequency. Furtherwheel speed increases cause the rotor shockfleld to move radiallyinwardly with the result being that MPT tones of varying level andfrequency appear to be excited and transmitted.

The second theory is normally designated the pressure field theory.Associated with each fan blade is a pressure field. The transmission ofthe pressure pulses as pure tones produces the multiple per revolutionpulses in the far field, i.e., NXr.p.m./60. Data recorded and reducedfrom transducers mounted close to the rotor of a highbypass fan clearlyshows -.the existence of these pulses at all fan speeds, i.e., bothbelow and above the fan speed at which the MPT's are measured in thefarfield, above a relative Mach number of one. At low fan 7 speeds, thesepressure pulses decay rapidly and do not reach the far field. At acertain fan speed a so-called "cutoff" point is reached at whichtheoretically the MPT pressure pulses propagate upstream and radiatefrom the inlet. The "cutoff" point for a front fan rotor engine occursat a tip'relative Mach number slightly greater than one (thiscoincidentally is also the speed at which the shock waves start toform). The pres sure pulses, therefore, would start to propagate abovethe c'u toff" fan speed. It is generally felt that the majority of theMPT tone energy, generated under either theory above, transm itsupstream in a region near the inlet duct wall. Noise suppression panelspositioned within the inlet duct wall can provide sound suppression overa broad frequency range, but for the most part are ineffective at lowerfrequencies (0 to 1,000 Hz.) where MPTs show maximum level.

SUMMARY OF THE INVENTION 'by providing the inlet duct wall of a turbofanengine with either an inflatable'surface which is capable of selectiveinflation or with mechanically actuated surface panels which are capableof increasing the velocity of the inlet flow near the inlet wall tosonic or near sonic conditions.

DESCRIPTION OF THE DRAWINGS While thespecificationconcludes with claimsparticularly pointing out and distinctly claiming the subject matter ofthe invention which is sought to be protected, an understanding of theinvention may be gained from the following detailed description of apreferred embodiment in connection with the accompanying drawings, inwhich:

FIG. 1 is a generally schematic sectional view of a high bypass,IGV-less turbofan engine;

FIG. 2 is an enlarged, sectional view of a portion of the inlet of theturbofan engine shown in FIG. 1;

FIG. 3 is an enlarged sectional view similar to FIG. 2 showing a noisesuppression device constructed in accordance with this invention in itsoperating position;

FIG. 4 is a graphical plot of far field noise generated by a typicalturbofan engine;

FIG. 5 is a plot, similar to FIG. 4, showing the effects of inlet wallsonic flow on MPT noise; and

FIG. 6 is a partial sectional view of an alternative embodiment.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawingswherein like numerals correspond to like elements throughout, FIG. 1shows in simplified fashion a gas turbine engine of the high bypassfront fan type comprising a core engine 12, which is essentially aturboshaft engine in that it includes a compressor 14, a combustor 16, agas generator turbine 18 for driving the compressor l4, and a powerturbine 20 arranged in axially spaced serial flow relationship. Theinner turbomachine, or core engine 12, is enclosed within a cylindricalcasing 22 which terminates at its downstream end in an exhaust nozzle 24through which the combustion products of the core engine I2 may bedischarged to produce thrust. To provide additional thrust, a fan 26 ismounted upstream of the core engine 12 and is driven by the powerturbine 20. The fan 26 is comprised of a plurality of fan blades 28which extends radially from a fan wheel '30, which is coupled forrotation to the power turbine 20. The fan blades 28 extend radiallyacross a bypass duct or passageway 32 defined between an outercylindrical casing 34 and a bullet nose" 36 located upstream of the fanblades 28. Downstream of the fan blades 28, the passageway 32 is splitinto two passages 38 and 40 by a splitter platfonn 42. Radiallypositioned between the casing 34 and the splitter platform 42 are aplurality of fan stator vanes 44, which are followed by a plurality offan outlet guide vanes 46. Thus, a portion of the airflow entering thepassageway 32 flows through the fan blades 28, into the passageway 38,through the stator vanes 44 and through the outlet guide vanes 46 andthereafter exists through an outlet opening 48 formed by the casing 34and the splitter platform 42. Since this air is pressurized in flowingthrough the fan blades 28, it provides forward thrust to the turbofanengine 10.

The remainder of the air flowing through the passageway 32 and the fanblades 28 enters the passageway 40. Located within this passageway 40are a plurality of inlet guide vanes 50 for the core engine 12, whichare followed by a plurality of rotatable compressor blades 52, whichextend from a compressor disc 54 and are coupled for rotation with thefan blades 28 by means of the disc 54 and a shaft 56. Located downstreamof the compressor blade 52 is a row of stator vanes 58. Air passingthrough the stator vanes 58 next flows into the core engine 12 throughthe passageway 60.

The gas turbine engine 10 is a high bypass ratio machine; by the termhigh bypass ratio" it is meant that the ratio of the mass flow of fluidin the bypass passageway 38 to the mass flow in the core engine 12 (orpassageway 40) is high. Since a substantial amount of energy isextracted from the combustion gases in driving the power turbine 20(except for losses, this energy is transferred to the bypass stream asthe air in the passageway 32 is accelerated by the fan blades 28), itwill occur to those skilled in the art that the major portion of thetotal thrust provided by the gas turbine engine 10 is generated by thebypass stream of air emanating from the outlet 48.

One characteristic of every high bypass ratio engine is that thediameter of the bypass fan is much larger than the diameter normallyassociated with a compressor or turbine section of the core engine.Because of this large diameter, the rotation of the fan 26 may result inunacceptable noise levels unless some provision is made for reducing theintensity of noise generated by such a fan. The present inventionrelates to such a device for reducing the noise level associated withrotation of the large diameter fan and, more particularly, relates toreducing multiple pure tone noise generated by such a fan.

Referring now to FIGS. 2 and 3, the details of applicant's noisereduction device are shown in an enlarged sectional view of the inlet ofthe turbofan engine I0. As shown therein, the outer casing 34 isprovided near its upstream end with an elastic diaphragm 62, whichpreferably surrounds the entire casing 34 on an inner wall 64 thereof.(While, for simplicity, reference hereinafter will be continually madeto the elastic diaphragm 62, it should be apparent to those skilled inthe art that mechanically operated panels could replace the diaphragmand applicants invention would operate in the same manner. The elasticdiaphragm 62 extends peripherally about the inner wall 64 of the casing34 and is secured thereto at its upstream and downstream ends in afluidically sealed manner. A conduit 66 is provided within the casing 34for supplying pressurized fluid from a source (not shown) of pressurizedair to the underside of the diaphragm 62. The conduit 66 is providedwith suitable control means such as a valve 68 for regulating the flowof pressurized fluid to the underside of the elastic diaphragm 62.

In its deflated position, as illustrated in FIG. 2, the diaphragm iscontiguous with the wall 64 so as not to interfere with the flow of airthrough the passageway 32. When pressurized fluid is allowed to pass tothe underside of the diaphragm 62 in order to inflate the same to theposition shown in FIG. 3, the diaphragm 62 extends into the passageway32 an insignificant amount as far as the total flow area of thepassageway 32 is concerned, but sufficiently far enough to increase thelocal velocity of airflow along the wall 64 as will presently bediscussed.

As previously mentioned, multiple pure tone noise is generated byrotation of the fan blades 28 either due to the transmission of thepressure pulses generated by each rotating blade 28 or due to thetransmission of shock waves generated near the tips of the rotatingblades 28. In either case, the majority of the pure tone noise istransmitted upstream along the inlet duct wall 64.

Referring to FIG. 4, a 50 Hz. narrow band spectrum of noise data isshown as recorded in the far field. As discussed above, MPT (pure tone)noise generated during certain portions of operation of the untreatedgas turbine engine 10 emanates from the inlet duct of the engine. SuchMPT noise shows up as a series of peaks, labeled MPT's, on FIG. 4.

Actuation of the inflatable diaphragm 62 by opening the valve 68 causesthe air at the inlet throat flowing near the inlet wall 64 to increasein velocity to sonic or near sonic conditions. Since sound cannot swim"upstream against sonic flow, the multiple pure tone noise is impeded andprevented from emanating from the inlet duct. The result of actuation ofthe diaphragm 62 to the point where flow along the wall 64 is betweenM=0.95 and 1.0 is plotted in FIG. 5. As can be seen, the number andintensity of the peaks has been drastically reduced.

As previously mentioned, a Kutney application, assigned to the sameassignee as the present invention, teaches the positioning of a similarinflatable diaphragm within an inlet duct of a turbofan engine. TheKutney inflatable diaphragm, however, cooperates with a platform memberto provide a nozzle which effectively chokes the entire outer passagewayof the turbofan engine shown in the Kutney application. The inflatablediaphragm 62 is not utilized to choke the flow within the inlet duct.That is, the inflatable diaphragm 62 expands only a sufficient amount toincrease the local velocity of flow along the inner wall 64 to sonic ornear sonic flow. In this manner, the small overall change in inletgeometry of the turbofan engine 10 will not affect the performance ofthe fan 26 in any significant manner. The relatively small physicalexpansion of the inflatable diaphragm 62 is sufficient, however, toprevent propagation of multiple pure tone noise energy from the inletduct. In this manner, possible annoyance to people located withinhearing of the turbofan engine is reduced.

While the inflatable diaphragm 62 would be capable of inflation duringany portion of the flight schedule, preferably inflation thereof willtake place only during takeoff conditions of an aircraft powered by thegas turbine engine 10. It is only during take off conditions thatmultiple pure tone noise constitutes a noise problem, as at higheraltitudes multiple pure tone noise has sufficiently subsided prior toreaching ground level as to not constitute a noise problem and at lowerapproach fan speeds multiple pure tone noise is not generated.

As shown in FIG. 6, the inflatable diaphragm 62 could be replaced by aseries of mechanically actuated panels -84,

which are capable of movement between the position shown in solid linesin FIG. 6 where they present a smooth inner wall 86, and the positionshown in dotted lines in FIG. 6 where they provide a suflicient changein inlet geometry to increase the velocity of flow near the inner wall86 to near sonic velocity. The center panel 82 would be constructed of aplurality of pieces positioned around the periphery and would includearms 87 to fit within grooves 88 provided in each of the panels 80 and84. The members making up the center panel 82 would also have to beprovided with sufficient spacing therebetween to allow for actuation. Anactuator 90 is provided for each of the panels 82 to selectively movethem to the dotted line position shown in FIG. 6. If desirable, thepanels 80-84 could be covered with a flexible cover or skin 92 toprovide a smooth flow surface.

While preferred embodiments of the present invention have been depictedand described above, it will be appreciated by those skilled in the artthat many modifications, substitutions, and changes may be made theretowithout departing from the inventions fundamental scheme. It isintended, therefore, that the appended claims cover such modifications.

We claim:

1. In a jet propulsion power plant of the turbofan type having a coreengine and a fan driven by the core engine, sound suppression meanscomprising:

a cylindrical casing enclosing the fan of said power plant and spacedfrom said core engine to fonn therewith an axially extending bypasspassageway for the passage of motive fluid between inlet and outletopenings at opposite ends of said casing,

expandable means carried by said casing adjacent said inlet opening,said expandable means having a first position wherein it is contiguouswith an inner wall of said casing and second position wherein it extendsinto said bypass passageway, and

means for selectively moving said expandable means between said firstand second positions, said second position adapted to increase thevelocity of flow nesr said inner wall to sonic or near sonic velocitywhereby multiple pure tone noise generated by said fan is prevented frompropagating forwardly through said inlet opening.

2. Sound suppression means as recited in claim 1 further characterizedin that said expandable means comprises a circumferential elasticdiaphragm and means are provided for inflating said diaphragm, saidinflating means comprising piping positioned within said casing andlying in fluidic flow cooperation with a source of pressurized fluid.

3. Sound suppression means as recited in claim 2 wherein said source ofpressurized fluid comprises said core engine.

4. Sound suppression means as recited in claim 3 further characterizedin that inflation of said diaphragm affects the velocity of only thatportion of the motive fluid flowing near said inner wall of said casing.

5. Sound suppression means as recited in claim 1 further characterizedin that said expandable means comprises a plurality of mechanicallyactuated panels.

6. Sound suppression means as recited in claim 5 wherein said panels arecovered by a flexible skin which is connected to said inner wall.

1. In a jet propulsion power plant of the turbofan type having a coreengine and a fan driven by the core engine, sound suppression meanscomprising: a cylindrical casing enclosing the fan of said power plantand spaced from said core engine to form therewith an axially extendingbypass passageway for the passage of motive fluid between inlet andoutlet openings at opposite ends of said casing, expandable meanscarried by said casing adjacent said inlet opening, said expandablemeans having a first position wherein it is contiguous with an innerwall of said casing and second position wherein it extends into saidbypAss passageway, and means for selectively moving said expandablemeans between said first and second positions, said second positionadapted to increase the velocity of flow near said inner wall to sonicor near sonic velocity whereby multiple pure tone noise generated bysaid fan is prevented from propagating forwardly through said inletopening.
 2. Sound suppression means as recited in claim 1 furthercharacterized in that said expandable means comprises a circumferentialelastic diaphragm and means are provided for inflating said diaphragm,said inflating means comprising piping positioned within said casing andlying in fluidic flow cooperation with a source of pressurized fluid. 3.Sound suppression means as recited in claim 2 wherein said source ofpressurized fluid comprises said core engine.
 4. Sound suppression meansas recited in claim 3 further characterized in that inflation of saiddiaphragm affects the velocity of only that portion of the motive fluidflowing near said inner wall of said casing.
 5. Sound suppression meansas recited in claim 1 further characterized in that said expandablemeans comprises a plurality of mechanically actuated panels.
 6. Soundsuppression means as recited in claim 5 wherein said panels are coveredby a flexible skin which is connected to said inner wall.